This invention relates to a method and apparatus for adjusting the shape of components of the eye and more particularly to making fixed changes in the corneal curvature to correct refractive error.
Deviations from the normal shape of the corneal surface produce errors of refraction in the visual process. The eye in a state of rest, without accommodation, focuses the image of distant objects exactly on the retina. Such an eye enjoys distinct vision for distant objects without effort. Any variation from this standard constitutes ametropia, a condition in which the eye at rest is unable to focus the image of a distant object on the retina. Hyperopia is an error of refraction in which, with the eye at rest, parallel rays from distant objects are brought to focus behind the retina. Divergent rays from near objects are focused still further back. In one aspect of hyperopia, the corneal surface is flattened which decreases the angle of refraction of rays as they pass through the refractive surfaces of the cornea, causing a convergence or focus of the rays at a point behind the retina. The retina is comprised partially of nerve fibers which are an expansion of the optic nerve. Waves of light falling on the retina are converted into nerve impulses and carried by the optic nerve to the brain to produce the sensation of light. To focus parallel rays on the retina, the hyperopic eye must either accommodate, i.e., increase the convexity of its lens, or a convex lens of sufficient strength to focus rays on the retina must be placed before the eye. Myopia (Gr. to squint) is that refractive condition in which, with accommodation completely relaxed, parallel rays are brought to focus in front of the retina. One condition which commonly causes myopia is when the corneal curvature is steepened, thus the refraction of rays is greater as the rays pass through the refractive surfaces of the cornea, and the over-refracted rays converge or focus in front of the retina in the vitreous of the eye. When the rays reach the retina they become divergent, forming a circle of diffusion and consequently a blurred image. A concave lens is used to correct the focus of the eye for myopia.
The normal treatment of these classic forms of refractive error of the eye is with the use of eyeglasses or contact lenses, both of which have well-known disadvantages to the user. It has been estimated that 60 million pairs of eyeglasses and 3 million pairs of contact lens are sold annually.
Recent research has been directed to operative techniques to change the refractive condition of the eye. Radial keratotomy (RK) is by far the most commonly used keratorefractive procedure to correct myopia. RK involves making equally spaced radial incisions in the peripheral cornea around a 3.0 to 5.0 mm diameter central, uncut clear zone. These incisions weaken the paracentral and peripheral cornea, which under the influence of intraocular pressure, causes outward bowing of the peripheral cornea, a compensatory flattening of the central cornea. This central corneal flattening then leads to a reduction in myopia. Many factors have been identified that affect the outcome of the RK procedure which the predictability is significantly less than achieved with eyeglasses and contact lens. See Mechanical Methods in Refractive Corneal Surgery, Flowers, Jr. and McDonnell, Current Opinions in Ophthalmology, 1994, 5; IV:81-89. Refinements in RK, and alternatives thereto, continue to evolve. Such techniques are generally referred to as "keratorefractive techniques". Two such techniques are more particularly called keratophakia and keratomileusis. Keratomileusis involves the regrinding of a corneal lamella into a meniscus or hyperopic lens to correct myopia or hyperopia. A corneal optical cryolathe has been especially developed for this procedure and is also used in the keratophakia procedure, when a homograft ground into a convex lens is placed interlamellarly to correct aphakic hypermetropia. The homograft tissue (corneal lamella) is frozen with carbon dioxide. The homograft is cut by the lathe as a contact lens would be, i.e., to the optical power required to effect the desired optical correction of the cornea. In keratomileusis, the anterior corneal lamella is shaped by the lathe. Further evaluation and explanation of these procedures can be found in the American Academy Ophthalmology, Ophthalmic Procedures Assessment Committee report approved Feb. 15, 1992, entitled: Keratophakia and Keratomileusis: Safety and Effectiveness. Another form of correction called "myopic keratomileusis in situ" is widely reported. See J Catatact Refrat. Surg., Vol. 17, July 1991, pages 424-435, Arena-Archila, et al. Myopic keratomileusis in situ: A preliminary report. Instead of modifying the stromal side of the resected disc (corneal lamella), the correction is made in the stromal bed. It is thus seen that present procedures in keratorefractive techniques are best limited to situations where other more standard corrective practices are found ineffective. It is readily seen that the limiting factors in such surgical techniques is the gross complexity involved not only with multiple incisions in corneal tissue for affecting the procedures but also complex suturing patterns, resulting in gross restructuring of the eye. The eye is thus faced with a difficult job of adjusting to this trauma.
Over the past few years developments have been made in the use of lasers as a means to reshape the cornea in an attempt to get rid of refractive errors. In these processes, pulsed lasers remove tissue from the cornea by shaving off or vaporizing portions of the corneal surface to cause it to flatten. The most common type is an Eximer laser. The fundamental effect of such a laser on tissue is a photochemical one, the breaking of molecular bonds with so much energy that the tissue fragments fly from the surface at supersonic speeds, leaving behind a discreet space. The process has been designated as ablative photodecomposition or photoablation. Complete explanation of "laser therapeutic keratectomy" can be found in Refractive Keratotoy, George O. Waring III, Mosby year Book, (1992) Chapter 19. The laser techniques are adaptable to be used in other corneal surgical techniques, including keratomileusis in-situ.
One of the problems with keratomileusis procedures is obtaining a smooth curvature upon the exposed stromal bed. See In Situ Myopic Keratomilesus Results in 30 Eyes at 15 Months, Bas & Nano, Refractive & Corneal Surgery, Vol. 7, pages 223-231, May/June 1991. Using laser techniques leave corrugated or rippled ablated surfaces, like a washboard. In addition the ablated surface becomes `work hardened` and contains a pseudo membrane of burned tissue that must be cleaned and cleared up.
As in all refractive correcting techniques, the risk benefit ratio is always considered individually and fully explained to prospective patients. See Corrective Measures for Myopia, Wilson and Keeney, Survey of Ohthalmology, Vol. 34, No. 4, Jan/Feb 1990, pages 294-304.